There are currently many different wireless communications systems promulgated by the telecommunications industries and used in the world. These systems are complex and they set forth specifications regarding all aspects of wireless communications, including physical characteristics of signal transmission, such as transmission frequency and operation mode.
One of the earliest wireless communication systems developed in North America is called the advanced mobile phone service ("AMPS"). Used for analog cellular communications, AMS specifies a mobile station transmission frequency band between 824 MHz and 849 MHZ. This band is often referred to as the 800 MHZ band or the cellular band. Within the same frequency band also operates a later developed system called the digital mobile phone service ("DMPS"), which is used for both digital and analog communications. These systems are generally referred to in the industry as AMPS 800 and DMPS 800.
A European wireless communication system, now being used in North America and other parts of the world, the global system for mobile communications ("GSM"), specifies a mobile station transmission frequency band between 890 MHZ to 915 MHZ and it is used for digital communications. Still another system, called personal communications system ("PCS") 1900 specifies a mobile station transmission frequency between 1850 MHZ and 1910 MHZ. The transmission frequency of PCS 1900 is substantially higher than that of AMPS 800 or GSM 800.
There exist many other cellular communications systems. For example, the nordic mobile telephone 450 system ("NMT-450) specifies a transmission frequency between 463 MHZ and 469 MHZ and the signal modulation technique of FDMA. The nordic mobile telephone 900 system ("NMY-900) specifies a transmission frequency between 935 MHZ and 960 MHZ and the same signal modulation technique.
As for digital cordless telephones, there are, for example, cordless telephone 2 ("CT2") requiring a transmission frequency between 864 MHZ and 868 MHZ and modulation technique of TDMA/FDM, and digital European cordless telephone ("DECT") specifying a transmission frequency between 1880 MHZ and 1900 MHZ with the same modulation technique.
Those different transmission frequency bands and operating modes present a unique challenge for wireless service providers and particularly for manufactures of wireless communications equipment. If a service provider wishes to replace its currently used wireless system with one operating in a higher frequency band (e.g. from AMPS 800 to PCS 1900), the existing base stations must be upgraded so that they operate in accordance with the new system. By using upconverters which convert a lower frequency signal to a higher frequency signal, the base stations can be upgraded to operate at a higher frequency signal, the base stations can be upgraded to operate at a higher frequency. Of course, the base stations must also be updated to comply with other aspects of the new wireless system.
In addition to upgrading the base stations, individual cellular telephones in the hands of customers must also be upgraded or replaced so that they be compatible with the new wireless system. In particular, since the power amplifier used in each cellular phone is optimized to operate within a particular frequency band and at a particular mode, it needs to be replaced with a new power amplifier suitable for operation under the new wireless standard.
For example, cellular phones used for AMPS 800 contain a power amplifier optimized to operate within the cellular band (i.e. the 800 MHZ band). If, however, AMPS 800 is replaced with PCS 1900, the old AMPS phones cannot be used any more, they must be upgraded or replaced. Replacement of cellular phones is expensive. A new cellular phone which can be easily upgraded is desired.
For cellular phone manufactures, different wireless systems require different power amplifiers, which increases cost. Different wireless systems present another problem. If a cellular phone user crosses from one area served by one wireless system into an area served by a different wireless system, he will not be able to use his phone. It is desired that the same cellular phone be used under different wireless systems and that the user can simply activate a switch to use it under a different wireless system. Preferably, when a user enters into an area served by different wireless system, the user's phone is automatically switched to operate under the new wireless system that covers the area. This can be achieved by a base station sensing a signal to the cellular phone to switch the cellular phone. In any event, it requires a power amplifier system capable of operating under different wireless systems.
MESFET power amplifiers for wireless applications generally require both a control circuit and a drain switch for controlling both the output power of the amplifier in an ON state, as well as to limit excessive leakage when in an OFF state. The proliferation of dual-band cell phones have required the use of dual-band power amplifiers. Single band MESFET power amplifiers often require a single drain switch. The control circuit of such an amplifier typically takes the form of a commercially available silicon MOSFET having a low Rds (drain to source resistance), and a silicon or gallium arsenide based negative voltage generator (nvg) and a control circuit.
Gallium arsenide-based control circuits typically take the form of a differential amplifying front end having a current biased level shifting buffer for translating input control signals (typically 0-3 volts) from a D/A converter to the negative MESFET voltage levels. Dual-band amplifiers (versus single band amplifiers) typically use two drain switches in control circuits; one for each power amplifier (PA) under control. The increased cost of a dual-band power amplifier which utilizes two drain switches and two negative voltage generators is a significant factor which prevents widespread use of such a device in a low cost dual-band application. Furthermore, the use of separate control circuits and drain switches provide an additional increase in the overall cost of the device by increasing the number of part insertions (for a phone manufacturer, for example), as well as increasing the overall phone board space for implementation.
In accordance with the present invention, there is provided a dual-band amplifier controller that utilizes only one drain switch and a single negative voltage generator. Because a dual-band telephone operates in only one band at any given time, only one power amplifier will be in operation (i.e. active) at any given time interval. That is, only one power amplifier will be drawing current from the battery at a time. This feature allows both power amplifiers to share a single drain switch, provided there is sufficient capacitive by-passing. Such a dual-band solution greatly simplifies the design, cost and complexity of a dual-band power amplifier.